The present invention relates to optical transmission members, and more particularly to members with minimized reflectivity. In a specific embodiment, the present invention relates to an improved glass faceplate of an image display cathode ray tube and a method of fabrication of such a glass faceplate with a minimized reflectivity. The invention finds general application with any optical member where the reflectivity is to be minimized.
The desirability of minimizing reflectivity from an optical surface is generally recognized, and particularly for image display devices which are utilized in high ambient background light. Thus, the present invention can be applied to a wide variety of optical members such as mirrors and lenses, and more specifically to image display cathode ray tubes, such as are used for instrumentation in aircraft cockpit displays. The basic phenomenon of reflectivity degrades the resolution and brightness of the displayed optical image or optical output as a result of the difference in indices of refraction at the interface between the optical member and air through which the image is displayed is viewed. It has been the practice to utilize antireflective surface coatings on such optical members. For vitreous glass faceplates, such as used in cathode ray display tubes, the most widely used antireflective coating is magnesium fluoride which is typically vapor or sputter deposited onto the glass surface at a high deposition temperature. The antireflective magnesium fluoride film thickness is controlled optically using light transmission or reflection from the member which is being coated. The typical vitreous glass faceplate has an index of refraction of about 1.5, and the index of refraction of magnesium fluoride is about 1.38, so that it is not entirely possible to impedance match the antireflective coating with the glass substrate member. A typical reflectivity from an uncoated optical member is about 4%, and the typical reduction in reflectivity had from antireflective coatings reduces the reflectivity to about 1% or greater.
The physical principles underlying the operation of single layer antireflective coatings are well-known and rely on destructive interference of reflected light waves from a system consisting of glass coated with a quarter wavelength antireflective coating, wherein the index of refraction of the coating is lower than that of the glass. For a theoretical impedance match the desired condition is index of refraction of the coating should equal the square root of the index of refraction of the glass. Materials simply do not exist which can meet this impedance matching requirement both with respect to the index of refraction for typical vitreous materials friom which optical members are formed, or with respect to the low index of refraction required of the antireflective coating material.
The most widely used antireflection material, magnesium fluoride while more resistant to environmental degradation than alternative coatings is still subject to scratching, or wearing as by oxidation or hydrolysis of the coating when exposed to the environment and to ultraviolet radiation. Other lower index of refraction materials such as sodium and potassium fluoride are less chemically stable and more easily physically damaged due to lack of mechanical strength.
The use of multiple layers of magnesium fluoride as a multiple layer antireflective coating has been used in attempts to further impedance match the multiple coatings to the optical substrate. However, such multiple coatings are costly and difficult to fabricate. A general review of the field of optical coating technology can be found in "Optical Coatings I, II, from the S.P.I.E. Proceedings in Vol. 50, 1975; and the S.P.I.E. Proceedings Vol. 140, 1978."
A recent innovation has been the use of ion bombardment or ion implantation to alter the surface or near surface regions of a variety of materials. Such an ion implanter as can be used in the semiconductor industry is set forth in U.S. Pat. No. 4,274,004, and U.S. Pat. No. 4,258,266. In general, such ion implantation systems direct a high energy stream of ions at a target which is typically disposed in a vacuum, with collisions of the ions with the target surface being such that the ions actually tunnel into or are implanted into the target material.